Process for the production of caustic potash and oxalic acid



Patented May 21, 1935 UNITED STATES PROCESS FOR THE PRODUCTION OF CAUS- TIC POTASH AND OXALIC ACID Max Enderli, Oestrich, Germany, assignor to firm Rudolph Koepp & Co. Ghemische Fabrik A. G., Oestrich, Germany, a corporation of Germany No Drawing. Application July 5, 1933, Serial No. 679,080. In Germany July 9, 1932 2 Claims.

5 to causticize potassium sulphate with caustic lime. 'This proposal has, however, not yielded a technically satisfactory result, because in the most favorable case yields of only about 40% could be obtained by the conversion and, in addition to more or less valueless by-products, only very dilute caustic potash solutions are obtained, from which the potassium hydroxide can only be recovered with correspondingly large evaporation costs. It has also been proposed to prepare caustic potash by interacting potassium salts with oxalic acid or ammonium oxalate. It has not been found possible to introduce these processes into practice, since they are attended by considerable losses and employ oxalic acid, which is very expensive and which could not be economically recovered within the sphere of these processes. x

It has now been found that, in contradistinction to the aforementioned process of the direct causticization of potassium sulphate, it is possible by a definite procedure to produce from potassium sulphate pure caustic potash in high yield and high concentration. Moreover, in contradistinction tothe aforementioned process employing oxalic acid, the valuable oxalic acid is not used up, but produced. This is efiected according to the invention by first producingpotassium formate from potassium sulphate and an alkaline earth metal formate or components yielding the same, and then converting the potassium formate by heating into potassium oxalate. The potassium oxalate is then converted with alkaline earth metal hydroxide into caustic potash solution and alkaline earth'metal oxalate. The resulting caustic potash solution is thereafter separated from the precipitated alkaline earth metal oxalate and concentrated according to requirements or worked up into solid caustic potash. The alkaline earth metal oxalate, for example calcium oxalate, however, is worked up to oxalic acid with mineral acids, for example sulphuric acid. It is thus possible, according to the process of the invention, to convert the relatively cheap starting materials, potassium sulphate, lime and carbon monoxide, into the more valuable products, caustic potash and oxalic acid.

The successful carrying out of this process is entirely unexpected. By the conversion of the potassium formate into potassium oxalate there is obtained, in addition to the extremely readily soluble potassium formate, the almost as readily soluble potassium oxalate. It would appear to be impossible to effect separation of these two salts without incurring losses of material. In addition it would be expected that, by the conversion of the potassium oxalate into caustic potash-and calcium oxalate, the latter would occlude considerable quantities of potassium salts, which would render the further working up of the same to oxalic acid diflicult and also give rise tolosses of potassium salts. Losses of potassium would, however, in all circumstances have prevented the process from being carried out economically and have rendered it impossible to carry the process into effect in practice, as

in the case of the known processes hereinbefore referred to.

The unexpected fact was, however, found that, when carrying the process into effect, the solubility of the potassium oxalate is reduced by the simultaneous presence of potassium formate and potash to such an extent that the potassium oxalate can be separated in a pure condition from the accompanying more soluble salts. It was, moreover, found that losses of potassium can be avoided both at this stage of the process, and also during the conversion of the potassium oxalate into caustic potash and calcium oxalate.

As hereinbefore mentioned potassium formate can be produced, within the ambit of the process of the invention, from potassium sulphate and alkaline earth metal formate. In this case the alkaline earth metal formate must be first produced from alkaline earth metal hydroxide and carbon monoxide or carbon monoxide-containing gases and then be caused to interact in the presence of water with the potassium sulphate. An alternative mode of procedure, however, is to produce the potassium formate directly from potassium sulphate, alkaline earth hydroxide and carbon monoxide or carbon monoxide-containing gases, for example by causing carbon monoxide or carbon monoxide-containing gases to interact with an aqueous mixture of potassium sulphate and alkaline earth metal hydroxide.

The resulting potassium formate solution is then first separated, for example by filtration, from the precipitated alkaline earth metal sulphate and, preferably after first removing any calcium salts still present, evaporated to dryness. The solid potassium formate is thereafter converted into potassium oxalate by heating, for example to temperatures between 300 and 450 C. The potassium oxalate is further treated with alkaline earth metal hydroxide, for example milk of lime, preferably at slightly elevated temperature and under stirring, and the resulting potash solution separated, for example by filtration, from the precipitated alkaline earth metal oxalate. The caustic potash solution may be concentrated in the Usual manner or, for the production of solidcaustic potash, be evaporatedito dryness. A mineral acid, for example sulphuric acid, is added to the alkaline earth metal oxalate and the resulting oxalic acid solution is separated from the precipitated alkaline earth-metal sul phate and worked up to oxalic acid.

Example 130 kgms. of calcium formate, which-may beof potassium sulphate in the presence of 1100.,

litres of water, the precipitated calcium sulphate filtered ofi and the filtrate, preferably after first removing anyv calciumsalts present, evaporated-to-dryness. The resulting potassium formate is converted into potassium oxalate by heatinglto temperatures of 300 to'450" C. About 130 kgms; of potassium oxalate are obtained, which are. causticized with 590 litres of a 10% milkof limeatabout 40? C. in a stirring apparatus. The resulting caustic potash solution is separatedfromthe calcium oxalate by'suctional filtration and: worked up to causticpotash by evaporating toy dryness, while the calcium oxalate is worked up to oxalic acid with sulphuric acid.

What I claim is:-

1. A process for the production of caustic potash and oxalic acid comprising the steps of producing potassium formate from potassium sulphate-and an alkaline earthmetal formate, converting the potassium formateby heating into potassium oxalate, converting the potassium oxalate with an alkaline earth metal hydroxide into potassium hydroxide and the corresponding alkaline earth metal oxalate, separating the resulting caustic potash solution from the alkaline earth metal oxalate and Working up the alkaline earth metal oxalate with sulphuric acid into oxalic acid;

2; A process for the production of caustic potasii and oxalic acid comprising the steps of producing potassium formate by interacting carbon monoxide with potassium sulphate and an alkaline earth metal hydroxide, converting the potassium formate by heating. into-potassium oxalate, converting the potassium oxalate withcan alkaline earth metalhydroxide into potassium hydroxide and thecorresponding alkaline earth metal oxalate, separating theresulting caustic potash solution from the alkaline earth metal oxalate'and working up the alkaline earth metal oxalate with sulphuricacid into oxalic acid.

MAX ENDERLI. 

